Re: Pointer to COM function
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X-Original-Date: Thu, 15 Nov 2007 12:45:16 +0100
X-Submission-Address: c++-submit@netlab.cs.rpi.edu
* lexxmt:
I going to port application from C to C++. I found one interesting
place that I dont't understand how to port to C++. Can anybody help
me ?
Some of this is going to be a little bit technology-specific, I'm
afraid, since your code is technology-specific.
I have COM object ISampleObject exported from different DLL, I have no
source for this library yet.
"DLL" is system-specific but happily that doesn't matter here: what it
means is just that you don't have the source code available, only a
header file and the binary level interface.
"COM" is an binary level ABI for the subset of C++ with no multiple
inheritance and no templates and only polymorphic classes, originally
only a Windows C and C++ technology, but subsequently variants of COM
have been adopted in a number of different systems. To achieve
consistent memory layout, every COM-compatible class must be derived
from a pure interface class named IUnknown. Other names are used in
COM-variants such as used in Mozilla technology and Linux GUI.
Here is original C code (without error handing e.g.)
...
void Register(IUnknown *obj)
IUnknown: see above.
{
ISampleObject *lpSampleObject;
obj-lpVtbl->QueryInterface(obj,&ISampleObject,&lpSampleObject);
This call doesn't seem to be correct: the second argument should be an
id (128-bit UUID) for the interface type you're requesting a pointer to.
Is the original code using a silly language extension where &T
provides that id for a type T?
Assuming a function interfaceId that provides that id,
template< class T > UUID const& interfaceIdOf();
template<> UUID const& interfaceIdOf<ISampleObject>{ return ...; }
the call translates to C++
obj->QueryInterface(
interfaceIdOf<ISampleObject>(),
reinterpret_cast<void**>( &lpSampleObject )
);
but you would be better off using a smart pointer that does the cast for
you -- as well as doing proper reference counting, i.e. doing AddRef
and Release calls automatically.
Several such smart pointers exist for COM.
At least some of them do the entire QueryInterface call for you
automatically when you specify the type of pointer you want.
RegisterCallBack(lpSampleObject,&lpSampleObject->lpVtbl-
CallbackFunction);
You cannot use a standard C++ member function pointer here: the
RegisterCallback function expects a pointer to a freestanding function.
You can either use platform-specific means (e.g. the C language
interface), or just provide a freestanding function:
void myStaticCallbackFunction( void* object, void* arg )
{
static_cast<ISampleObject*>( object )->onCallback(
*static_cast<Whatever*>( arg )
);
}
...
RegisterCallBack( lpSampleObject, &myStaticCallbackFunction );
Cheers, & hth.,
- Alf
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